Fluorocarbon based fluids have found widespread use in industry for refrigeration applications such as air conditioning and heat pump applications.
Vapor compression is one type of refrigeration. In its simplest form, vapor compression involves changing the refrigerant from the liquid to the vapor phase through heat absorption at a low pressure and then from the vapor to the liquid phase through heat removal at an elevated pressure. First, the refrigerant is vaporized in the evaporator which is in contact with the body to be cooled. The pressure in the evaporator is such that the boiling point of the refrigerant is below the temperature of the body to be cooled. Thus, heat flows from the body to the refrigerant and causes vaporization. The formed vapor is then removed by means of a compressor in order to maintain the low pressure in the evaporator. The temperature and pressure of the vapor are then raised through the addition of mechanical energy by the compressor. The high-pressure vapor then passes to the condenser whereupon heat exchange with a cooler medium, the sensible and latent heats are removed with subsequent condensation. The hot liquid refrigerant then passes to the expansion valve and is ready to cycle again.
While the primary purpose of refrigeration is to remove energy at low temperature, the primary purpose of a heat pump is to add energy at higher temperature. Heat pumps are considered reverse cycle systems because for heating, the operation of the condenser is interchanged with that of the refrigeration evaporator.
Certain chlorofluoromethane and chlorofluoroethane derivatives have gained widespread use in refrigeration applications including air conditioning and heat pump applications owing to their unique combination of chemical and physical properties.
The majority of refrigerants utilized in vapor compression systems are either single components fluids or azeotropic mixtures. Single component fluids and azeotropic mixtures are characterized as constant-boiling because they exhibit isothermal and isobaric evaporation and condensation. The use of azeotropic mixtures as refrigerants is known in the art; for example, see R. C. Downing, FLUOROCARBON REFRIGERANTS HANDBOOK, Prentice-Hall, 1988 and U.S. Pat. Nos. 2,101,993 and 2,641,579.
R-502 is an azeotropic blend Which consists of monochlorodifluoromethane (R-22) and chloropentafluoroethane (R-115), a fully halogenated chlorofluorocarbon. R-502 has been routinely used for medium to low temperature refrigeration applications and has a boiling point of about -50.1.degree. F. (-45.6.degree. C.).
Azeotropic or azeotrope-like compositions are desired because they do not fractionate upon boiling. This behavior is desirable because in the previously described vapor compression equipment with which these refrigerants are employed, condensed material is generated in preparation for cooling or for heating purposes. Unless the refrigerant composition exhibits a constant boiling point, i.e. is azeotrope-like, fractionation and segregation will occur upon evaporation and condensation and undesirable refrigerant distribution may act to upset the cooling or heating.
Non-azeotropic mixtures have been disclosed as refrigerants for example in U.S. Pat. No. 4,303,536 but have not found widespread use in commercial applications even though the potential of non-azeotropic refrigerant blends to exhibit improved thermodynamic performance has often been discussed in the literature; see for example T. Atwood, "NARBS--The Promise and the Problem", American Society of Mechanical Engineers, Winter Annual Meeting, Paper 86-WA/HT-61, 1986 and M. O. McLinden et al., "Methods for Comparing the Performance of Pure and Mixed Refrigerants in the Vapour Compression Cycle". Int. J. Refrig. 10, 318(1987).
A blend of HFC-125 and HFC-143a was disclosed as having utility as a refrigerant in RESEARCH DISCLOSURE 15402, Feb. 1977 but this disclosure implied that such a blend was non-azeotropic. i.e. would fractionate upon evaporation or condensation, and stated that the blend was disadvantageous because it was flammable.
The use of non-azeotropic mixtures which fractionate during the refrigeration cycle introduces additional complexity into the system which necessitates hardware changes. The use of non-azeotropic refrigerants has been avoided primarily due to the added difficulty in charging and servicing refrigeration equipment and the situation is further complicated if an inadvertent leak in the system occurs during use or during service. The composition of the mixture could change affecting system pressures and system performance. If one component of the non-azeotropic mixture is flammable, then fractionation could shift the composition into the flammable region with potential adverse consequences.
U.S. Pat. No. 4,810,403 teaches that certain nonazeotropic blends of three or more halocarbon components maintain a substantially constant vapor pressure even after evaporative losses of up to 50 percent of the original refrigerant charge. In other words, the mixtures exhibit a certain degree of constant-boiling behavior even though they are non-azeotropic.
The art is continually seeking new fluorocarbon based azeotrope-like mixtures which offer alternatives for refrigeration and heat pump applications. Currently, of particular interest, are fluorocarbon based azeotrope-like mixtures which are considered to be environmentally safe substitutes for the presently used fully halogenated chlorofluorocarbons(CFC's). The latter are suspected of causing environmental problems in connection with the earth's protective ozone layer.
The substitute materials must also possess those properties unique to the CFC's including chemical stability, low toxicity, non-flammability, and efficiency in-use. The latter characteristic is important in refrigeration and air-conditioning especially where a loss in refrigerant thermodynamic performance or energy efficiency may have secondary environmental impacts through increased fossil fuel usage arising from an increased demand for electrical energy. Furthermore, the ideal CFC refrigerant substitute would not require major engineering changes to conventional vapor compression technology currently used with CFC refrigerants.
Mathematical models have substantiated that hydrofluorocarbons, such as pentafluoroethane(HFC-125) and 1,1,1-trifluoroethane(HFC-143a) will not adversely affect atmospheric chemistry, being negligible contributors to ozone depletion and to green-house global warming in comparison to the fully halogenated species. Although chlorodifluoromethane(HCFC-22) contains a chlorine atom in its molecular structure, it also contains a hydrogen atom and as such, its atmospheric lifetime is much lower than that of a fully halogenated CFC and it has a significantly lower ozone depletion potential.
Because HFC-143a is as efficient as R-502 and provides a modest increase in refrigeration capacity. HFC-143a might be considered a good refrigerant substitute for R-502. HFC-143a has a boiling point of about -53.7.degree. F.(-47.6.degree. C.). However, a disadvantage of HFC-143a as a refrigerant is that the vapor of HFC-143a is flammable. As a result, the shipping, handling, and use of HFC-143a have to be carefully controlled due to the potential flammability.
Because HFC-125 is nonflammable and provides a modest increase in refrigeration capacity compared with R-502, HFC-125 might be considered a good refrigerant substitute for R-502. HFC-125 has a boiling point of about -55.5.degree. F.(-48.5.degree. C.). However, a disadvantage of HFC-125 is that HFC-125 is about 5% less efficient than R-502.
HCFC-22 is used in most residential and light commercial air-conditioning, some industrial low-temperature requirements, and some medium- and low-temperature display cases in supermarkets. HCFC-22 has a boiling point of about -41.4.degree. F.(-40.8.degree. C.). Although HCFC-22 offers improved coefficient of performance(COP) compared with R-502, the compressor discharge temperature of the refrigerant is much greater for HCFC-22 than for R-502. It is known in the art that excessive compressor discharge temperatures have a deleterious effect on compressor reliability. Some compressor manufacturers recommend that discharge temperatures be kept below 225.degree.-250.degree. F.(107.degree.-121.degree. C.).
As such, a refrigerant which has a low ozone depletion potential and is a negligible contributor to green-house global warming compared with fully halogenated CFC refrigerants, is nonflammable, has a COP and capacity comparable to that of R-502, and has a low compressor discharge temperature is needed in the art.
It is an object of this invention to provide novel azeotrope-like compositions based on pentafluoroethane; 1,1,1-trifluoroethane; and chlorodifluoromethane which will not fractionate under normal cooling or heating conditions.
Another object of the invention is to provide novel environmentally acceptable refrigerants for use in the aforementioned applications.
Other objects and advantages of the invention will become apparent from the following description.